An oil well borehole which is being prepared for oil and/or gas production generally includes a steel casing supported by a cement casing in the annulus between the outside wall of the steel casing and the walls of the borehole. The cement casing isolates two or more zones such as, for example, a production zone from an aqueous brine zone. Typically, a number of perforations are formed in the casing and through the cement in the production zone thereby providing fluid communication between the formation and the well. A production well string provides a fluid conduit through which the produced oil or gas travels to the surface. A portion of the production string opposite the casing perforations is referred to as the screen. The screen is made of tubing with numerous holes or slots formed in the tubing wall. Wire is then typically wrapped around the tubing so as to achieve a desired mesh which permits the formation products to flow up the production string but blocks the passage of undesirable solid materials entrained in the oil or gas.
One of the more serious problems encountered in producing well fluids is the presence of formation solids, particularly sand, in the produced fluid. Because of the high fluid pressures involved, there is a sand blasting effect on the screen which can quickly lead to premature erosion and failure of the screen and tubing.
A common technique used to overcome the effect of erosion by formation sand is to pack gravel in the casing perforations and in the annulus around the screen within the casing. The gravel acts as a filter which blocks the passage of formation sand to the screen but which permits the products such as oil and gas to permeate to the screen and flow up the production string.
The so-called gravel pack comprises gravel mixed with water and other materials such as thickening agents, suspension agents and the like and is pumped as a slurry down the well and into the formation at the perforations. The gravel must be effectively packed in order to prevent voids which would limit its effectiveness as a filter medium. When packed under pressure, the slurry dehydrates with the carrier fluid being returned to the surface through a wash pipe.
The gravel packing process is carried out using a packer apparatus and a service tool. Generally, the packer is an apparatus which, in normal use, is placed in the well and directs the slurry to flow to the desired location for gravel packing. The packer performs this task by separating the annulus between the production string and the casing in two sealed regions, i.e., the upper annulus above the packer and the lower annulus which is below the packer. The packer is provided with a plurality of slips which can be hydraulically actuated to bite into the casing to locate and support or set the packer against the casing in the annulus. A plurality of packer sealing elements are then compressed and/or expanded radially outwardly against the casing to effect the seal between the upper and lower annuli.
The hydraulic actuation of the packer and its sealing elements is effected by the use of a service tool which may also be referred to as a running tool or a crossover tool. The service tool is connected to the packer, typically by threaded engagement, and both tools are run simultaneously into the well with a work string. The service tool provides a conduit in conjunction with the work string tubing for hydraulically setting the packer and, additionally, provides crossover ports for carrying the gravel slurry from the tubing outwardly into the lower annulus through openings or squeeze ports in the tool. In normal use, the service tool is removed from the well after the packing operation is completed and the packer remains set in the well. After the service tool is removed, the production string can be run into the well and formation fluids can be withdrawn through the production string to the surface.
The manner in which the service tool has been connected to and released from the packer is generally accomplished in one of two ways. In one type of packer/service tool connection, the service tool is in threaded engagement with corresponding threads on the packer prior to insertion into the wellbore. After the use of this type of service tool has been completed, it is released from the packer by applying torque to the service tool in order to release the threaded engagement with the packer. In another type of packer/service tool release mechanism such as described in U.S. Pat. No. 4,660,637, a fluid passage in the service tool is blocked such as by a ball which is conveyed to the service tool through the tubing and which seats in a ball seat provided in the service tool. A high pressure is then applied against the ball and ball seat to a point at which shear pins coupling the service tool with the packer are sheared thereby releasing the service tool for retrieval.
A ball seat and tubing-conveyed ball are also used in such packer/service tool arrangements to close off reverse circulation of well fluids (up the tubing); for allowing hydraulic pressure setting of the packer; to effect the opening of the gravel packing crossover ports so that fluid communication between the service tool tubing and the lower annulus to be gravel packed is made and, finally, to release the service tool from the packer for retrieval. In all cases, such ball sealing is effected by inserting the ball at the surface and allowing it to gravitate pumping it through the tubing to the ball seat.
After seating of the ball, all of the various operations of setting the packer, opening the crossover ports and release from the packer are effected by stepped increases in the fluid pressure against the seated ball. Obviously, precise control of the pressure and pressure increases and avoidance of high pressure pulses is critical to the proper operation of the packer/service tool assembly.
Additional problems with the above arrangement can arise due to improper seating or "loss" of the tubing-conveyed sealing ball. The passage of the ball through the tubing can be arrested such as by hanging up in a deviated hole at a tubing bend. As hole deviations approach horizontal, the problems with tubing-conveyed sealing balls increases to the point of near total unreliability that the ball will reach and proper seat so that the various pressure-activated procedures may be performed. An additional problem with a tubing-conveyed sealing ball involves the time it takes for passage through the tubing to the seat which can take as long as several hours in a deep and/or deviated well.